The present invention relates to a method for reducing the chemical conversion of oxidizable or disproportionable materials contacting carbonaceous chars.
It is well-known that certain oxidizable or disproportionable compounds contacting carbonaceous chars such as charcoals and activated carbons can be chemically decomposed or converted into other chemical entities. Some examples of these types of chemical conversions include the conversion of hydrogen sulfide, sulfur dioxide, phosphine, arsenite, nitrite, and iron(II), in the presence of O2 and H2O, into other oxidized species such as sulfur, sulfuric acid, phosphoric acid, arsenate, nitrate, and iron(III), respectively. Organic compounds may be similarly affected. For example, the oxidation of ascorbic acid, hydroquinone, and oxalic acid in contact with a carbonaceous char have also been reported. In some cases, disproportionation, or autoxidation/reduction, of the compound may occur upon contact with the char. Such reactions are typical for compounds such as hydrogen peroxide, hydrazine, and monochloramine.
Carbonaceous chars such as activated carbons, especially those produced at high temperatures (i.e., above 700xc2x0 C.), often possess catalytic properties which facilitate the chemical conversion of such materials. The origins of this catalytic activity can usually be attributed to two principal factors: (1) the inorganic non-carbon ash constituents of the char, e.g. iron, potassium, and calcium, and (2) the inherent catalytic properties of the carbon itself. If the char is used for the physical adsorption, removal, and/or recovery of substances from gas or liquid media, the presence of catalytic reactivity in the char may lead to an unwanted chemical conversion of compounds in the process media into materials that contaminate the final process stream or that interfere with the physical adsorption process. Moreover, the reactions which lead to catalytic chemical conversion are often highly exothermic, increasing the likelihood of ignition of the char itself.
Examples of processes in which the catalytic activity of the char is a liability include the purification of disproportionable or oxidizable substances such as hydrogen peroxide and organic glycols, respectively, and the adsorption, recovery, and re-use of oxidizable solvents such as acetone and methyl ethyl ketone. In these types of applications, the primary component of the process stream is itself a material which can become reactive if catalyzed by the carbon under certain conditions. In other applications it may be desirable to preserve a reactive constituent of the process stream, e.g. hydrogen peroxide, such that it is available for reaction with species other than the carbon itself. Less obvious is the need for a non-catalytic, adsorptive carbon for applications in which the potentially reactive component is an interference, constituting only a small part of the overall adsorbate loading. For example, in streams containing small concentrations of an oxidizable material such as hydrogen sulfide and much higher concentrations of an adsorbable, recoverable organic, it is possible that the oxidation of the hydrogen sulfide, which is only weakly physically adsorbed per se, can cause a build-up over time of highly adsorbable and polar reaction products, such as sulfuric acid, which can greatly interfere with the adsorption of the organic components of the stream. If the inherently reactive carbon catalytic sites can be deactivated, more effective use can be made of the physical adsorption properties of the carbon for the removal and recovery of the organic stream components.
A number of prior art methods used to reduce the catalytic activity of materials in contact with carbonaceous chars have been directed to the addition of chemical agents to the process media or changes in the process conditions. For example, it is known to reduce the disproportionation of hydrogen peroxide in contact with a carbonaceous char by lowering the pH of the process stream. Other prior art methods to reduce the chemical conversion of materials in contact with the char address the removal of catalytically-active ash constituents from the char. For example, it is known to remove acid-soluble ash constituents by acid-washing the char. After treatment, the char is rinsed with water and/or an aqueous solution of a base to remove and/or neutralize the acid. Hydrofluoric acid solution, an extremely hazardous material, has been found to be particularly effective for removing such ash components. Ash reductions up to or exceeding an order of magnitude are usually achievable by these methods. Other ash removal processes treat the char first with caustic solution to remove caustic-soluble components, followed by the aforementioned acid extraction treatment to remove the remaining acid-soluble components.
In other embodiments of the prior art, the ash components are treated with agents such as silanes to reduce their catalytic reactivity without removing them from the carbon surface. In all of these prior art methods, however, little regard has been given to the inherent reactivity of the carbon itself, which alone may be sufficient to interfere with the intended function of the char.
Where the reactivity of the carbon has been considered, no methods have been identified or reported which can deactivate the carbon effectively and largely irreversibly. For example, it has been observed that oxygen can chemisorb to the carbon surface and cause the carbon to become less catalytically active. However, after use, and upon high-temperature thermal treatment of the char to remove other adsorbed materials, it is found that the oxygen is also lost from the carbon. Upon removal of the oxygen, the inherent catalytic reactivity of the carbon once again becomes manifest, requiring additional post-treatment of the thermally-treated char to deactivate the carbon prior to re-use. Therefore, the oxygen appears to have merely masked, and not destroyed, the catalytic activity of the carbon.
Accordingly, it is the object of the present invention to provide a method which reduces the chemical conversion of oxidizable or disproportionable materials contacting carbonaceous chars in liquid or gaseous process media by reducing the inherent catalytic activity of the carbon itself. It is further the object of the present invention to provide this reduction of chemical conversion without the addition of chemical agents to the process media or changes in the process conditions. Additionally, it is the object of the present invention to provide a method for the deactivation of the carbon in so that the deactivation is largely irreversible upon high-temperature thermal treatment of the char.
Generally, the present invention comprises a method for reducing the chemical conversion of oxidizable or disproportionable materials in liquid or vapor process media which contact a carbonaceous char. The process does not involve the addition of deactivating agents to the liquid or vapor media. In the present invention the nature of the carbonaceous char is altered such that the inherent reactivity of the carbon itself is eliminated or greatly reduced.
The present invention provides contacting the carbonaceous char with a non-nitrogen-containing Bronsted acid, such as hydrochloric acid, at temperatures at or below the boiling point of the acid or aqueous acid solution and thereafter heating the treated char to temperatures above 750xc2x0 C. Nitrogen-containing acids such as nitric-acid-are not generally preferred in the present invention since treatment by such acids is known to increase the catalytic activity of carbonaceous chars. The amounts of acid used in the present invention are such that the char contains preferably 1-10 millimoles of acid per mole of carbon prior to raising the temperature of the acid-containing carbon to or above 750xc2x0 C. In addition it may also be desirable to reduce any non-carbon contributions to the overall reactivity of the char, but is not necessary for those applications requiring only the deactivation of the carbon itself. If the char is rinsed with water following acid treatment, the rinse is carried out such that the pH of the rinse water is less than 5 to ensure that sufficient acid hydrogen is present on the char to accomplish the required deactivation of the catalytic carbon sites upon raising the temperature.
Upon exposure of the acid-treated char to temperatures at or above 750xc2x0 C., the deactivation of the carbon reactivity is largely effected. The heating of the acid-treated carbon is carried out preferably in an inert or oxygen-free atmosphere, while the cooling of the deactivated char to ambient temperatures may be carried out in an oxygen-containing atmosphere to impart additional deactivation properties to the char according to the methods of known prior art. xe2x80x9cSuch oxygen-containing atmosphere may include any oxidant containing oxygen, phosphorus, sulfur, or a halogen.xe2x80x9d
Other prior art methods for deactivating other reactive elements of the char may also be used in combination with the method of the present invention to impart still other desirable properties to the deactivated char. For example, for chars containing high levels of inorganic ash constituents, the ash components may be removed with base and/or acid after, as well as prior to, application of the method of the present invention.
The method of the present invention may be repeated any number of times with the same char to achieve additional levels of deactivation. However, for chars with comparatively low levels of initial inherent carbon reactivity, a single application of the method is generally sufficient to achieve acceptable reductions in carbon reactivity and, therefore, significant reductions in the amount of decomposition or chemical conversion of materials which come into contact with the carbon. Other advantages of the present invention will become apparent from a perusal of the following detailed description of presently preferred embodiments of the invention.